March 07, 2011

Origin of Life from Outer Space ?

Overcoming Bias links to a paper and commentary about the possible discovery of cyanobacteria-like fossils in three meteorites. The paper, by Richard B. Hoover, Ph.D. NASA/Marshall Space Flight Center makes the argument that the discovered structures don't appear to be recent earthly contaminants.

I won't comment on the validity of the case, but I will give my €0.02 on the "Origin of Life" question, on which there are two main theories:
  • The view, popularized by Carl Sagan in Cosmos and also found in the Selfish Gene about life beginning with a single replicator molecule forming by chemical accident in primordial Earth, this molecule eventually leading (via evolution) to all extant life on Earth.
  • The idea of panspermia of Hoyle and Wickramasinghe that life is common in the universe, that it can travel through space in things like meteors, and that it arrived on Earth from space.
As a non-expert, I see that both ideas have their advantages:
  • The Earthly Origin idea is simple, as we know that life exists on Earth, so why posit its origin from elsewhere?
  • The Panspermia hypothesis is also simple in a Copernican principle sort of way: the universe is large, so why posit that life originated here just because it is found here?
How can we weigh the relative advantages of the two hypotheses? The Earthly Origin assumes that:
  1. The formation and survival of the first replicator on Earth is not too improbable, and it could have happened in less than a billion years between the formation of the Earth and the earliest attestation of life.
  2. Transport and survival through space of a replicator is improbable
If we think about it:
  • the probability that a replicator will emerge in a given volume of space increases with that volume and must approach unity for the entire universe (because life does exist)
  • The probability that it will reach Earth from a certain distance decreases with that distance.

So, it is a matter of weighing in the probabilities: a replicator may spontaneously form throughout the Universe, but how likely is it to spread from its point of origin across space and reach us?

The various commentaries on the paper make some interesting points, and I will add some of my own:
  1. If we could date the origin of earthly life (using some type of molecular clock) and find it to be older than the geological age of the Earth, that would favor panspermia. A commentator states that this is actually twice the age of the earth, but I am too skeptical of molecular clocks across such time scales to put much faith in that claim, and one math-less and one-under-review study are offered as evidence.
  2. If we found extra-terrestrial life that would also favor panspermia, provided we exclude the possibility of contamination; Hoover's paper presents evidence that this is the case.
  3. On the other hand it is possible that ancient-looking extra-terrestrial life fossils falling out of the sky in the 19th century could in fact be due the nostos of bacteria ejected from the Earth during its formative period billions of years ago.
  4. If life isn't found in the solar system then the argument for panspermia would be weakened, because presumably Earth is not the only world that could have been hit by life-bearing meteorites, nor the only world where it might flourish. So far, no extra-terrestrial life has been found in the solar system.
  5. If we could show the spontaneous generation of a replicator in the lab, that would be evidence for Earthly Origin; of course, no lab is large enough and no experiment long-running enough to statistically exclude this possibility.
  6. We could also show the perseverence of life through experiment, by e.g., sending a probe or commandeering an asteroid and setting it on a highly elliptical trajectory that would bring it back to Earth's vicinity in a century or so: seed it with bacteria and see what comes back during its next pass from Earth's vicinity. Again, this might show that the transport of life is possible across "short" distances, but not that it is possible across interstellar space.
I always carry a small basket when it comes to alien life claims from NASA-affiliated scientists, but it's certainly interesting to think about the Origin of Life, regardless of the fate of this particular paper (which I'm sure will be torn to pieces by skeptics, if it hasn't been already by the time you read this).

My personal opinion is that the idea of panspermia would be cool if true, but Fermi's Paradox is a bit difficult to reconcile with a life-filled universe, unless we accept that life is common but EM-transmitting intelligence not.

PS: The fact that the Journal of Cosmology seems to be a web-based effort certainly makes me a tiny bit suspicious, and a 10-min perusal of its contents does suggest that it publishes "unconventional" content. On the other hand, I like this bit from its editor who seems legit:
Official Statement from Dr. Rudy Schild,
Center for Astrophysics, Harvard-Smithsonian,
Editor-in-Chief, Journal of Cosmology.

Dr. Richard Hoover is a highly respected scientist and astrobiologist with a prestigious record of accomplishment at NASA. Given the controversial nature of his discovery, we have invited 100 experts and have issued a general invitation to over 5000 scientists from the scientific community to review the paper and to offer their critical analysis. Our intention is to publish the commentaries, both pro and con, alongside Dr. Hoover's paper. In this way, the paper will have received a thorough vetting, and all points of view can be presented. No other paper in the history of science has undergone such a thorough analysis, and no other scientific journal in the history of science has made such a profoundly important paper available to the scientific community, for comment, before it is published. We believe the best way to advance science, is to promote debate and discussion.
That certainly agrees with my general philosophy on peer review. Whatever the fate of the paper, putting it out there for criticism by all is a praiseworthy attitude and certainly a better guarantee of scrutiny than a handful of Nature or Science reviewers.


PS said...

As far as I'm concerned, the Panspermia hypothesis has never been any more than an attempt to put off answering the really fundamental question, which is how did life first appear? That's the question that science has so far been unable to answer, so saying it came from somewhere else is just a way of avoiding the question.

Dienekes said...

You are right that science has been unable to answer the question of life's first appearance. The usual explanation is that it was a rare chemical event that created a replicator which subsequently evolved to the most recent common ancestor of all life. It would be nice if there was a plausible chemical probability calculation, say, how probable is the spontaneous formation of an evolvable replicator in so much volume of material X, Y, Z in so much time. That is obviously not an easy thing to do convincingly, because it is difficult to calculate probabilities for extreme events. Without such calculation, the question of life's origin here or elsewhere pretty much comes down to people's prior beliefs.

I personally see no reason to favor either hypothesis very strongly.

Leslie Schwartz said...

Life, the pre-cursors to life could have alternatively developed both on earth and extra-terrestrially.

There is evidence of pre-cursors, if not life itself in Mars rocks that made their way to earth, and in space, such as chlorophyll being found in "space" between the planets.

Given the vast range of environments life has been found in various forms on earth, I have concluded that life is likely much more prevalent than scientists, academics have presumed for past decades.

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